THIS PHOTO from the U.S. Geological Survey shows a core sample of mud containing methane hydrate, ice-like crystals that form at low temperatures and high pressure, taken from the ocean floor off the coast of Southern California. Scientists have discovered a large undersea deposit of this frozen methane, but whether it can be harnessed as a potential energy source is unknown. AP photo

LOS ANGELES (AP) -- Scientists have discovered an undersea deposit of frozen methane just off the Southern California coast, but whether it can be harnessed as a potential energy source is unknown.

In recent years, there has been a growing interest in tapping methane hydrates, ice-like crystals that form at low temperatures and high pressure in seabeds and in Arctic permafrost.

Scientists estimate that the methane trapped in previously known frozen reservoirs around the globe could power the world for centuries. But finding the technology to mine such deposits has proved elusive.

The newly discovered deposit, believed to be substantial in size, was found about 15 miles off the coast at a depth of about 2,600 feet, at the summit of an undersea mud volcano. Scientists were conducting an unrelated study when they came across the volcano, which sits on top of an active fault zone in the Santa Monica Basin.

The discovery is detailed in the February issue of the journal Geology.

The ecosystem surrounding the methane hydrate site was unlike any of the other vast hydrate deposits around the world. Scientists found seashells and clams with unique chemical characteristics, suggesting the area experiences an extreme flux of methane gas mixing with water, said Jim Hein, a marine geologist at the U.S. Geological Survey in Menlo Park.

In additional to technical problems standing in the way of mining methane hydrates, Hein said mining this deposit probably would be difficult because of its proximity to shipping lanes from Los Angeles and Long Beach.

Some scientists also worry about the environmental effects of such large-scale gas deposits. Hydrates are estimated to contain about three times as much methane as is currently in the atmosphere, and some scientists say releasing it could lead to global warming and change the world's climate.